The x-ray free electron laser (XFEL) enables probing a highly compressed material response at the subnanosecond timescale. We exploit the ultrafast XFEL pulse to combine reflection x-ray diffraction and laser-driven shock compression to perform a study of the phase transformation and stability in Fe and Fe-Si alloys. Our approach enables us to observe that solid-solid phase transformations occur in Fe and Fe-${\mathrm{Si}}_{8.5\mathrm{wt}\%}$ in $\le 130$ ps at $\sim 130$ GPa; no transformation is observed in Fe-${\mathrm{Si}}_{16\mathrm{wt}\%}$ up to 110 GPa. Density functional theory calculations predict similar phase relations.

Extensive fractional quantum Hall effect (FQHE) has been observed in graphene-based materials. Some of the observed fractions are anomalous in that FQHE has not been established at these fractions in conventional GaAs systems. One such fraction is $4/13$, where incompressibility has recently been reported in graphene [Kumar et al., Nat. Commun. 9, 2776 (2018)]. We propose a partonic wave function at $4/13$ and show it to be a viable candidate to describe the Coulomb ground state. Using the effective edge theory, we make predictions for experimentally measurable properties of the state.

We introduce an adaptive-weighted tree tensor network for the study of disordered and inhomogeneous quantum many-body systems. This Ansatz is assembled on the basis of the random couplings of the physical system with a procedure that considers a tunable weight parameter to prevent completely unbalanced trees. Using this approach, we compute the ground state of the two-dimensional quantum Ising model in the presence of quenched random disorder and frustration, with lattice size up to $32\times 32$. We compare the results with the ones obtained using the standard homogeneous tree tensor networks and the completely self-assembled tree tensor networks, demonstrating a clear improvement of numerical precision as a function of the weight parameter, especially for large system sizes.

We investigate quench dynamics across many-body localization (MBL) transition in an interacting one-dimensional system of spinless fermions with aperiodic potential. We consider a large number of initial states characterized by the number of kinks $N_{\mathrm{kinks}}$ in the density profile, such that the equal number of sites are occupied between any two consecutive kinks. We show that on the delocalized side of the MBL transition the dynamics becomes faster with increase in $N_{\mathrm{kinks}}$ such that the decay exponent $\gamma$ in the density imbalance increases with increase in $N_{\mathrm{kinks}}$. The growth exponent of the mean square displacement which shows a power-law behavior $\langle x^2\left(t\right)\rangle \sim t^{\beta }$ in the long-time limit is much larger than the exponent $\gamma$ for one-kink and other low-kink states though $\beta \sim 2\gamma$ for a charge density wave state. As the disorder strength increases ${\gamma }_{N_{\mathrm{kink}}}\to 0$ at some critical disorder, $h_{N_{\mathrm{kinks}}}$, which is a monotonically increasing function of $N_{\mathrm{kinks}}$. A one-kink state always underestimates the value of the disorder at which the MBL transition takes place but $h_{\text{1}\text{kink}}$ coincides with the onset of the subdiffusive phase preceding the MBL phase. This is consistent with the dynamics of interface broadening for the one-kink state. We show that the bipartite entanglement entropy has a logarithmic growth $aln\left(Vt\right)$ not only in the MBL phase but also in the delocalized phase and in both the phases the coefficient $a$ increases with $N_{\mathrm{kinks}}$ as well as with the interaction strength $V$. We explain this dependence of dynamics on the number of kinks in terms of the normalized participation ratio of initial states in the eigenbasis of the interacting Hamiltonian.

The gap function $\mathrm{\Delta }\left(\omega \right)$ carries essential information on both, the pairing glue as well as the pair-breaking processes in a superconductor. Unfortunately, in nearly localized superconductors with a nonconstant density of states in the normal state, the standard procedure for extraction of $\mathrm{\Delta }\left(\omega \right)$ cannot be applied. Here we introduce a model-independent method that makes it possible to extract $\mathrm{\Delta }\left(\omega \right)$ also in this case. The feasibility of the procedure is demonstrated on the tunneling data for the disordered thin films of TiN. We find an unconventional feature of $\mathrm{\Delta }\left(\omega \right)$ which suggests that the electrons in TiN are coupled to a very soft pair-breaking mode.

We report the observation of strong magnetic proximity coupling in the heterostructure superconductor ${\mathrm{Sr}}_2{\mathrm{VO}}_{3-\delta }\mathrm{FeAs}$, determined by upper critical field $H_{c2}\left(T\right)$ measurements up to 65 T. Using the resistivity and the radio-frequency measurements for both $H\parallel ab$ and $H\parallel c$, we found a strong upward curvature of $H_{c2}^c\left(T\right)$, together with a steep increase in $H_{c2}^{ab}\left(T\right)$ near $T_c$, yielding an anisotropic factor ${\gamma }_H=H_{c2}^{ab}/H_{c2}^c$ up to $\sim 20$, much higher than those of other iron-based superconductors. These are attributed to the Jaccarino-Peter effect, rather than to the multiband effect, due to strong exchange interaction between itinerant Fe spins of the FeAs layers and localized V spins of Mott-insulating ${\mathrm{SrVO}}_{3-\delta }$ layers. These findings provide evidence for strong antiferromagnetic proximity coupling that is comparable to the intralayer superexchange interaction of the ${\mathrm{SrVO}}_{3-\delta }$ layer and sufficient to induce magnetic frustration in ${\mathrm{Sr}}_2{\mathrm{VO}}_{3-\delta }\mathrm{FeAs}$.

High-entropy carbides (HECs) show excellent mechanical properties, but the strengthening mechanism remains unclear. In this work, using first-principles calculations, we comparatively investigated the ideal mechanical properties of $\left(\mathrm{HfNbTaTiZr}\right)\mathrm{C}$ and its constituent transition metal carbides (TMCs), as well as their sensitivity to point defects. It was found that the properties near the equilibrium point follow the rule of mixture, while those far from the equilibrium state are governed by the weakest link. Most importantly, the introduction of carbon vacancy significantly weakens the mechanical properties of the TMCs, while it less affects and even improves the mechanical properties of the HEC, indicating that the insensitivity to point defects may be one of the significant features of HECs. The results showed that the interaction between lattice distortion and defects would play an important role in improving the mechanical properties of HECs, which may provide guidance for the composition design of HECs.

We report the effect of high pressure on the superconducting, vortex pinning, and structural properties of a polycrystalline, noncentrosymmetric superconductor ${\mathrm{Re}}_6\mathrm{Hf}$. The superconducting transition temperature,$T_c$, reveals a modest decrease as pressure $\left(P\right)$ increases with a slope of −0.046 K/GPa (−0.065 K/GPa) estimated from resistivity measurements up to 8 GPa (magnetization measurement ∼1.1 GPa). Structural analysis up to ∼18 GPa reveals monotonic decreases of lattice constant without undergoing any structural transition and a high value of bulk modulus $B_0\approx 333.63$ GPa, indicating the stability of the structure. Furthermore, the upper critical field and lower critical field at absolute temperature [$H_{c2}\left(0\right)$ and $H_{c1}\left(0\right)$] decrease slightly from the ambient pressure value as pressure increases up to 2.5 GPa. In addition, analysis of $\rho \left(T,H\right)$ up to $P\sim \hspace{0.17em}2.5$ GPa using thermally activated flux flow of vortices revealed a double linearity field dependence of activation energy of vortices $\left[U_o\left(H\right)\right]$, confirming the coexistence of single and collective pinning vortex states. Moreover, analysis of critical current density using the collective pinning theory showed the transformation of $\delta T_c$ to $\delta l$ pinning as pressure increases, possibly due to migration of grain boundaries. Besides, the band structure calculations using density functional theory show that density of states decreases modestly with pressure, which may be a possible reason for such a small decrease in $T_c$ by pressure.

Odd-frequency Cooper pairs are gathering attention for the convenience of investigating the edge state of topological superconductors including Majorana fermions. Although a spinless $p$-wave superconductor has only one Majorana fermion in a topological phase, the system with magnetic fields can reach the topological phases with multiple Majorana fermions. To distinguish these multiple Majorana fermion phases, we correlate the energy spectrum with the odd-frequency pair amplitude as increasing the system size. The system size dependence tells us three pieces of information: the parity of the number of the Majorana fermions at the edge, the number of low-energy modes corresponding to the Majorana fermions with different localization lengths, and the fingerprints of the Majorana fermions. Also, we present the spatial dependence of the odd-frequency $\mathrm{f}$ vector that is created from odd-frequency pair amplitude and the spin structure of odd-frequency Cooper pairs. We find that the odd-frequency $\mathrm{f}$ vector is fixed in the same direction in any topological phase. Also, we show that the spin state of odd-frequency Cooper pairs tends to be oriented toward the direction of the magnetic fields. Our results highlight that the odd-frequency Cooper pairs can be a good indicator for the detection of the multiple Majorana fermions and the distinction among the topological phases.

A quantitative description of the excited electronic states of point defects and impurities is crucial for understanding materials properties, and possible applications of defects in quantum technologies. This is a considerable challenge for computational methods, since Kohn-Sham density functional theory (DFT) is inherently a ground-state theory, while higher-level methods are often too computationally expensive for defect systems. Recently, embedding approaches have been applied that treat defect states with many-body methods, while using DFT to describe the bulk host material. We implement such an embedding method, based on Wannierization of defect orbitals and the constrained random-phase approximation approach, and perform systematic characterization of the method for three distinct systems with current technological relevance: a carbon dimer replacing a B and N pair in bulk hexagonal BN (${\mathrm{C}}_{\text{B}}{\mathrm{C}}_{\text{N}}$), the negatively charged nitrogen-vacancy center in diamond (${\mathrm{NV}}^{-}$), and an Fe impurity on the Al site in wurtzite AlN (${\text{Fe}}_{\text{Al}}$). In the context of these test-case defects, we demonstrate that crucial considerations of the methodology include convergence of the bulk screening of the active-space Coulomb interaction, the choice of exchange-correlation functional for the initial DFT calculation, and the treatment of the “double-counting” correction. For ${\mathrm{C}}_{\text{B}}{\mathrm{C}}_{\text{N}}$ we show that the embedding approach gives many-body states in agreement with analytical results on the Hubbard dimer model, which allows us to elucidate the effects of the DFT functional and double-counting correction. For the ${\mathrm{NV}}^{-}$ center, our method demonstrates good quantitative agreement with experiments for the zero-phonon line of the triplet-triplet transition. Finally, we illustrate challenges associated with this method for determining the energies and orderings of the complex spin multiplets in ${\text{Fe}}_{\text{Al}}$.

In this work we prove that the one-body position operator for periodic systems that we have recently proposed [Phys. Rev. B 99, 205144 (2019)] is unique modulo a phase factor and an additive constant. The proof uses several general physical constraints that a periodic one-body position operator should satisfy. We show that these constraints are sufficient to uniquely define a position operator that is compatible with periodic boundary conditions.

It was recently shown that Aharonov-Bohm (AB) cages exist for quantum walks (QW) on certain tilings—such as the diamond chain or the dice (or ${\mathcal{T}}_3$) lattice—for a proper choice of coins. In this article, we probe the robustness of these AB cages to various perturbations. When the cages are destroyed, we analyze the leakage mechanism and characterize the resulting dynamics. Quenched disorder typically breaks the cages and leads to an exponential decay of the wave function similar to Anderson localization. Dynamical disorder or repeated measurements destroy phase coherence and turn the QW into a classical random walk with diffusive behavior. Combining static and dynamical disorder in a specific way leads to subdiffusion with an anomalous exponent controlled by the quenched disorder distribution. Introducing interaction to a second walker can also break the cages and restore a ballistic motion for a “molecular” bound state.

In this work, we study the in-plane optical phonon modes of current-carrying single-layer graphene whose coupling to the $\pi$ electron gas is strong. Such modes are expected to undergo a frequency shift compared to the non-current-carrying state due to the nonequilibrium occupation of the Dirac cone electronic eigenstates with the flowing $\pi$ electron gas. Large electron-phonon coupling can be identified by an abrupt change in the slope of the phonon mode dispersion known as the Kohn anomaly, which mainly occurs for (i) the in-plane longitudinal optical (LO) and transverse optical (TO) modes at the Brillouin zone (BZ) center ($\mathrm{\Gamma }$ point), and (ii) the TO modes at the BZ corners ($K$ points). We show that the breaking of the rotational symmetry by the dc current results in different frequency shifts to the $\mathrm{\Gamma }$-TO and $\mathrm{\Gamma }$-LO modes. More specifically, the dc current breaks the TO-LO degeneracy at the $\mathrm{\Gamma }$ point which ideally would be manifested as the splitting of the Raman G peak.

We propose a new kind of geometric effective theory based on curved space-time single valley Dirac theory with spin connection for twisted bilayer graphene under generic twist angle. This model can reproduce the nearly flat bands with the particle-hole symmetry around the first magic angle. The bandwidth is near the former results given by Bistritzer-MacDonald model or density matrix renormalization group. Even more, such geometric formalism allows one to predict the properties of rotating bilayer graphene which cannot be accessed by former theories. As an example, we investigate the Bott index of rotating bilayer graphene. We relate this to the two-dimensional Thouless pump with quantized charge pumping during one driving period which could be verified by transport measurements.

The interaction of a femtosecond optical pulse with a ${\mathrm{Fe}}_1/({\mathrm{MgO})}_3$(001) metal/oxide heterostructure is investigated using time-dependent density functional theory calculations in the real-time domain. We systematically study electronic excitations as a function of laser frequency, peak power density, and polarization direction. While spin-orbit coupling is found to result in only a small time-dependent reduction of magnetization (less than 10%), we find a marked anisotropy in the response to in-plane and out-of-plane polarized light, which changes its character qualitatively depending on the excitation energy: the Fe layer is efficiently addressed at low frequencies by in-plane polarized light, whereas for frequencies higher than the MgO band gap, we find a particularly strong response of the central MgO layer for cross-plane polarized light. For laser excitations between the charge transfer gap and the MgO band gap, the interface plays the most important role, as it mediates concerted transitions from the valence band of MgO into the $3d$ states of Fe closely above the Fermi level and from the Fe states below the Fermi level into the conduction band of MgO. As these transitions can occur simultaneously without altering the charge balance of the layers, they could potentially lead to an efficient transfer of excited carriers into the MgO bulk, where the corresponding electron and hole states can be separated by an energy which is significantly larger than the photon energy.

Recent advances in many-body physics have made it possible to study correlated electron systems at the two-particle level. In dynamical mean-field theory (DMFT), it has been shown that the metal-insulator phase diagram is closely related to the eigenstructure of the susceptibility. So far, this situation has been studied using accurate but numerically expensive solvers. Here, the iterated perturbation theory (IPT) approximation is used instead. Its simplicity makes it possible to obtain analytical results for the two-particle vertex and the DMFT Jacobian. The limited computational cost also enables a detailed comparison of analytical expressions for the response functions to results obtained using finite differences. At the same time, the approximate nature of IPT precludes an interpretation of the metal-insulator transition in terms of a Landau free energy functional.

Motivated by the recent experimental realizations of hyperbolic lattices in circuit quantum electrodynamics and the research interest in the non-Euclidean generalization of topological phenomena, we investigate the Chern insulator phases in a hyperbolic $\{8,3\}$ lattice, which is made from regular octagons (8-gons) such that the coordination number of each lattice site is 3. Based on the conformal projection of the hyperbolic lattice into the Euclidean plane, i.e., the Poincaré disk model, by calculating the Bott index ($B$) and the two-terminal conductance, we reveal two Chern insulator phases (with $B=1$ and $B=-1$, respectively) accompanied by quantized conductance plateaus in the hyperbolic $\{8,3\}$ lattice. The numerical calculation results of the nonequilibrium local current distribution further confirm that the quantized conductance plateau originates from the chiral edge states and the two Chern insulator phases exhibit opposite chirality. Moreover, we explore the effect of disorder on topological phases in the hyperbolic lattice. It is demonstrated that the chiral edge states of Chern insulators are robust against weak disorder in the hyperbolic lattice. More fascinating is the discovery of disorder-induced topologically nontrivial phases exhibiting chiral edge states in the hyperbolic lattice, realizing a non-Euclidean analog of a topological Anderson insulator. Our work provides a route for the exploration of topologically nontrivial states in hyperbolic geometric systems.

We study the dynamics of phase defects trapped in a finite optically imprinted ring lattice in binary polariton condensates, under the influence of the cross interaction (CI) between the condensate fractions in different spin components and the spin-orbit interaction (SOI). In this configuration, we find that a vortex circulates unidirectionally in optically induced orbits because of the Magnus force acting in the polariton fluid and the vortex' angular velocity is influenced by the SOI and CI. Interestingly, in our system, these two interactions can also lead to elongated and frozen phase defects, forming a frozen dark solution with similarity to a dark soliton but with finite size in both spin components. When the dark solution is stretched further to occupy the entire orbit of a condensate ring, the phase defect triggers a snake instability and induces the decay of the dark ring solution. The circulation direction of a single vortex is determined by the Magnus force. This situation is more complex for the group motion of multiple vortices because of significant vortex-antivortex interaction. The collective motion of such vortex constellations, however, is determined by the SOI.